Add PEP518/pyproject.toml file

PEP 518 introduces a way of specifying that stuff should be installed before try to run the setup.py. Currently when installing pygobject via pip, pygobject fails to build unless pycairo is installed beforehand (it does eventually install via setup.py install, but this means a wheel isn't build). By adding pycario to the requirements for building, when pip 10 comes out soon, the build will work the first time (so a wheel is always built).